You can ask him to increase the orfice size of the humidifier.
That mod should have a long enough on time to maintain the humidity level in your home.
What were the outdoor temps, when you were down to 15%RH.
Generally though. It sounds more like an excess of fresh air infiltration is causing your humidity problem.
Where is your duct work located?
Also. The YTH9421(IAQ thermostat) would have been a much better thermostat for that equipment configuration.